The internal heat density of electronic equipment has kept increasing year by year with recent increase in the speed or integration of circuits of electric or electronic equipment, and increase in the packaging density of heat-generating electronic components in printed circuit boards. Therefore, there is a demand for members having high thermal conductivity and electrical insulating properties and efficiently dissipating heat generated in electronic components and the like.
Thermosetting resins, such as epoxy resins, for use in the insulating layers of printed circuit boards, have low thermal conductivity in themselves. Accordingly, a method of adding to a thermosetting resin larger amount of an inorganic filler excellent in thermal conductivity is known in order to improve thermal conductivity as printed circuit boards. However, the addition of the larger amount of the inorganic filler to the thermosetting resin composition deteriorates moldability due to the decreased volume ratio of the thermosetting resin and is more likely to cause cracks or voids between the resin and the inorganic filler. Therefore, there are problems of deteriorated heat resistant properties upon moisture absorption, decreased elastic modulus, and reduced copper foil peel strength due to insufficient close contact between the resin and the inorganic filler. In light of such problems, use of various resin compositions has been proposed.
For example, Patent Literature 1 discloses a resin composition comprising a naphthol aralkyl-based cyanic acid ester resin and an epoxy resin, and discloses that the resin composition supplemented with a specific amount of an inorganic filler exerts excellent heat resistance, thermal conductivity and water absorbability.
Patent Literature 2 discloses a resin composition comprising a cyanate compound and an epoxy resin, and discloses that the resin composition supplemented with two types of inorganic fillers differing in particle size has favorable moldability and exerts high heat dissipating properties, a high glass transition temperature, copper foil peel strength, and heat resistance upon moisture absorption.
Patent Literature 3 discloses a resin composition comprising an epoxy resin and a curing agent, and discloses that the resin composition supplemented with borate particles coated with hexagonal boron nitride as an inorganic filler exerts a high glass transition temperature, copper foil peel strength, heat resistance upon moisture absorption, flame retardancy, a low coefficient of thermal expansion and high heat dissipating properties.